Sorting

Insertion Sort

Build the sorted prefix one item at a time, shifting larger values right until the current key can be inserted.

Algorithm

The checked-in replay follows the same small input and final output across all 21 DSA books, so this Fortran DSA implementation can be compared directly with the other languages.

Basic Implementation

basic.f90 
program sort_insertion
    implicit none
    integer :: arr(5) = [5, 1, 4, 2, 8]
    integer :: i, j, key
    do i = 2, 5
        key = arr(i)
        j = i - 1
        do while (j >= 1 .and. arr(j) > key)
            arr(j + 1) = arr(j)
            j = j - 1
        end do
        arr(j + 1) = key
    end do
    print '(*(I0,1X))', arr
end program sort_insertion

Complexity

  • Time: O(n^2) worst and average, O(n) best
  • Space: O(1)
  • Stable: yes

Implementation notes

  • Keep the explicit algorithmic steps instead of calling a standard-library sort. The replay is meant to expose comparisons, movement, and recursion.
  • The implementation is intentionally compact for learning and replay, not a production sorting utility.
sorted prefix Positions before the scan index are already sorted.
shifting Larger values move one slot right to make room for the key.